The present invention relates to a method for the preparation of vitreous hollow microspheres from a vitreous volcanic deposit as the starting material. More particularly, the invention relates to an efficient method for the preparation of vitreous hollow microspheres having high mechanical strengths and excellent whiteness from a naturally occurring volcanic deposit.
Vitreous hollow microspheres as an industrial material are widely employed and the consumption thereof is rapidly increasing in recent years as a light-weight filler in various kinds of metallic materials, ceramics, concretes, plastics and the like by virtue of their very low particle density and excellent heat resistance.
Japanese Patent Publication 48-17645 discloses a method for the preparation of vitreous hollow microspheres from a vitreous volcanic deposit as the starting material, in which a vitreous volcanic sandy deposit called "shirasu" occurring in southern Kyushu Island of Japan is subjected to calcination at a temperature of 800.degree. to 1200.degree. C. for a length of time of 10 seconds to 10 minutes followed by elutriation in water or by air classification of particles to obtain vitreous hollow microspheres. This method, however, is not applicable to the processing of vitreous volcanic deposits having a particle diameter not exceeding 20 .mu.m as a starting material of vitreous hollow microspheres as desired.
On the other hand, Japanese Patent Publication 2-296750 discloses a method for the preparation of ultrafine vitreous hollow microspheres in which a hydrothermal heating treatment of the starting material is undertaken by the use of an acid solution. This method, however, is defective because the hydrothermal treatment proceeding at a temperature higher than 100.degree. C. must be undertaken in an acid-resistant and pressure-resistant reaction vessel necessarily accompanied by an economical problem due to the high investment for the facilities if not to mention the very low efficiency of the treatment.